We use \textit{ab initio} calculations to estimate formation energies of cation (transition metal) antisite defects at oxide interfaces and to understand the basic physical effects that drive or suppress the formation of these defects. Antisite defects are found to be favored in systems with substantial charge transfer across the interface, while Jahn-Teller distortions and itinerant ferromagnetism can prevent antisite defects and help stabilize atomically sharp interfaces. Our results enable identification of classes of systems that may be more and less susceptible to the formation of antisite defects and motivate experimental studies and further theoretical calculations to elucidate the local structure and stability of oxide interface systems.
YFeO$_3$ thin films are a recent addition to the family of multiferroic orthoferrites where Y\textsubscript{Fe} antisite defects and strain have been shown to introduce polar displacements while retaining magnetic properties. Complete control of the multiferroic properties, however, necessitates knowledge of the defects present and their potential role in modifying behavior. Here, we report the structure and chemistry of antiphase boundaries in multiferroic YFeO$_3$ thin films using aberration corrected scanning transmission electron microscopy combined with atomic resolution energy dispersive…
Single-phase multiferroic materials that allow the coexistence of ferroelectric and magnetic ordering above room temperature are highly desirable, motivating an ongoing search for mechanisms for unconventional ferroelectricity in magnetic oxides. Here, we report an antisite defect mechanism for room temperature ferroelectricity in epitaxial thin films of yttrium orthoferrite, YFeO3, a perovskite-structured canted antiferromagnet. A combination of piezoresponse force microscopy, atomically resolved elemental mapping with aberration corrected scanning transmission electron microscopy and density…
The defect-mediated hexagonal boron nitride (hBN) supercell display the visible optical spectra and electronic characteristics. The defects in the hBN supercell include the atomic vacancy, antisite, antisite vacancy, and substitution of a foreign atom for boron or nitrogen. The hBN supercell with VB, CB, and NB-VN has the high electron density of states across the Fermi level which indicates the high conductive electronic characteristics. The hBN with defects including atomic vacancy, antisite vacancy, and substitution of a foreign atom for boron or nitride exhibit the distinct spin-resolved o…
Disorder, primarily in the form of oxygen vacancies, cation stoichiometry and atomic inter-diffusion, appear to play vital roles in the electronic and transport properties of the metallic electron liquid at the oxide hetero-interfaces. Antisite disorder is also understood to be a key player in this context. In order to delineate the roles of two of these key factors, oxygen vacancy and antisite disorder, we have investigated the effect of oxygen vacancy on the antisite disorder at a number of interfaces separating two La-based transition metal oxides using density functional theory. Oxygen vac…
A systematic study of crystal structure, local structure, magnetic and transport properties in quenched and temper annealed Ni$_{2-x}$Mn$_{1+x}$Sn alloys indicate the formation of Mn$_3$Sn type structural defects caused by an antisite disorder between Mn and Sn occupying the Y and Z sublattices of X$_2$YZ Heusler structure. The antisite disorder is caused by the substitution of Ni by Mn at the X sites. On temper annealing, these defects segregate and phase separate into $L2_1$ Heusler and $D0_{19}$ Mn$_3$Sn type phases.
Local modes caused by defects play a significant role in the thermal transport properties of thermoelectrics. Of particular interest are charge-neutral defects that suppress thermal conductivity, without significantly reducing electrical transport. Here we report a temperature dependent Raman study, that identifies such a mode in a standard thermoelectric material, Bi2Te2Se. One of the modes observed, whose origin has been debated for decades, was shown mostly likely to be an antisite defect induced local mode. The anomalous temperature independent broadening of the local mode is ascribed to t…
The influence of implantation-induced point defects (PDs) on SiC oxidation is investigated via molecular dynamics simulations. PDs generally increase the oxidation rate of crystalline grains. Particularly, accelerations caused by Si antisites and vacancies are comparable, and followed by Si interstitials, which are higher than those by C antisites and C interstitials. However, in the grain boundary (GB) region, defect contribution to oxidation is more complex, with C antisites decelerating oxidation. The underlying reason is the formation of a C-rich region along the oxygen diffusion pathway t…
Lattice defects affect the long-term stability of halide perovskite solar cells. Whereas simple point defects, i.e., atomic interstitials and vacancies, have been studied in great detail, here we focus on compound defects that are more likely to form under crystal growth conditions, such as compound vacancies or interstitials, and antisites. We identify the most prominent defects in the archetype inorganic perovskite CsPbI$_3$, through first-principles density functional theory (DFT) calculations. We find that under equilibrium conditions at room temperature, the antisite of Pb substituting Cs…
BAs was predicted to have an unusually high thermal conductivity at room temperature of 2000$\,$Wm$^{-1}$$\,$K$^{-1}$, comparable to that of diamond. However, the experimentally measured thermal conductivity of BAs single crystals is an order of magnitude lower. To identify the origin of this large inconsistency, we investigated the lattice structure and potential defects in BAs single crystals at atomic scale using aberration-corrected scanning transmission electron microscopy (STEM). Rather than finding a large concentration As vacancies ($V_\mathrm{As}$), as widely thought to dominate the t…
We perform ab-initio calculations on Bi$_\mathrm{Se}$ antisite defects in the surface of Bi$_2$Se$_3$, finding strong low-energy defect resonances with a spontaneous ferromagnetism, fixed to an out-of-plane orientation due to an exceptional large magnetic anisotropy energy. For antisite defects in the surface layer, we find semi-itinerant ferromagnetism and strong hybridization with the Dirac surface state, generating a finite energy gap. For deeper lying defects, such hybridization is largely absent, the magnetic moments becomes more localized, and no energy gap is present.
Ferroelectric belongs to an important class of materials showing very interesting nonlinear optical properties that have a variety of application in photonic devices In a discrete Hamiltonian, if we consider the space and time dependence of polarization vectors with an interaction term between two polarization domains, it gives rise to a nonlinear Klein-Gordon (K-G) equation that has been shown in some systems [1]. This equation as a governing equation enables multiple time-scale analysis (MTSA) to be performed on a real ferroelectric material that reveals new type of information on both linea…
Using scanning tunneling microscopy and spectroscopy, we visualized the native defects in antiferromagnetic topological insulator $\mathrm{MnBi_2Te_4}$. Two native defects $\mathrm{Mn_{Bi}}$ and $\mathrm{Bi_{Te}}$ antisites can be well resolved in the topographic images. $\mathrm{Mn_{Bi}}$ tend to suppress the density of states at conduction band edge. Spectroscopy imaging reveals a localized peak-like local density of state at $\sim80$~meV below the Fermi energy. A careful inspection of topographic and spectroscopic images, combined with density functional theory calculation, suggests this re…
We report first-principles calculations that clarify the formation energies and charge transition levels of native point defects and carbon clusters in the 4H polytype of silicon carbide (4H-SiC) under a carbon-rich condition. We applied a hybrid functional that reproduces the experimental bandgap of SiC well and offers reliable defect properties. For point defects, we investigated single vacancies, antisites, and interstitials of Si and C on relevant sites. For carbon clusters, we systematically introduced two additional C atoms into the perfect 4H-SiC lattice with and without removing Si ato…
Being atomically thin and amenable to external controls, two-dimensional (2D) materials offer a new paradigm for the realization of patterned qubit fabrication and operation at room temperature for quantum information sciences applications. Here we show that the antisite defect in 2D transition metal dichalcogenides (TMDs) can provide a controllable solid-state spin qubit system. Using high-throughput atomistic simulations, we identify several neutral antisite defects in TMDs that lie deep in the bulk band gap and host a paramagnetic triplet ground state. Our in-depth analysis reveals the pres…
Determining defect types and concentrations remains a big challenge of semiconductor materials science. By using ab-initio thermal conductivity calculations we reveal that Ni/vacancy antisites, and not the previously claimed Sn/Zr antisites, are the dominant defects affecting thermal transport in half-Heusler compound ZrNiSn. Our calculations correctly predict the thermal conductivity dependence with temperature and concentration, in quantitative agreement with published experimental results. Furthermore, we find a characteristic proportionality between phonon-antisite scattering rates and the…
Defect Engineering for Stabilizing Magnetic and Topological Properties in Mn(Bi1-xSbx)2Te4
Fudan University（复旦大学）；Southeast University（东南大学）；Eastern Institute of Technology（宁波工程学院）；Shanghai Jiao Tong University（上海交通大学）；East China Normal University（华东师范大学）；Chinese Academy of Sciences（中国科学院）
Thermoelectric and Magnetic Properties in Doped Fe$_2$VAl within a Bipolar Random Anderson Model
Department of Applied Physics, Tokyo University of Science（东京理科大学应用物理系）；Research Institute for Science and Technology, Tokyo University of Science（东京理科大学科学技术研究院）
The importance of inversion disorder in the visible light induced persistent luminescence in Cr$^{3+}$ doped AB$_2$O$_4$ (A = Zn or Mg and B = Ga or Al)
Cr $^{3+}$掺杂的尖晶石化合物AB $_2$ O $_4$（A=Zn，Mg，B=Ga，Al）在紫外或X射线激发下表现出近红外长余辉发光。此外，ZnGa $_2$ O $_4$和在较小程度上MgGa $_2$ O $_4$的持续发光也可以通过Cr $^{3+}$的$^4$ A $_2$ $ \rightarrow $ $^4$ T $_2$跃迁的可见光激发来诱导，这使得这些化合物适合作为小动物体内光学成像的生物标记物。我们将这种特殊的光学性质与ZnGa $_2$ O $_4$和MgGa $_2$ O $_4$中存在的反位缺陷相关联。通过使用X射线吸收精细结构（XAFS）光谱，与电子顺磁共振（EPR）和光发射光谱，它表明，反位缺陷浓度的增加，结果在Cr-O键长和八面体晶体场能量的减少。一部分的缺陷是在Cr $^{3+}$离子的封闭环境中，如所示的EPR和XAFS峰的应变增宽后观察到的反位无序增加。与ZnGa $_2$ O $^{3+}$和ZnGa $_2$ O相比，ZnAl $_2$ O $_4$在可见光激发下显示出最大的晶场分裂和最小的反位无序，但似乎没有显示出相当大的持续发光。这些结果突出了Cr $^{3+}$离子与相邻的反位缺陷在Cr $^{3+}$掺杂的AB $_2$ O $_4$尖晶石化合物所表现出的持久发光机制中的重要性。
We present a first-principles computational study of cation-Se $Σ$3 (112) grain boundaries in CuGaSe$_2$. We discuss the structure of these grain boundaries, as well as the effect of native defects and Na impurities on their electronic properties. The formation energies show that the defects will tend to form preferentially at the grain boundaries, rather than in the grain interiors. We find that in Ga-rich growth conditions Cu vacancies as well as Ga at Cu and Cu at Ga antisites are mainly responsible for having the equilibrium Fermi level pinned toward the middle of the gap, resulting in car…
The effect of a variety of intrinsic defects and defect clusters in bulk and thin films of SrTiO$_3$ on ferroelectric polarization and switching mechanism is investigated by means of density-functional-theory (DFT) based calculations and the Berry phase approach. Our results show that both the titanium Ti$_\mathrm{Sr}^{\bullet \bullet}$ and strontium Sr$_\mathrm{Ti}^{"}$ antisite defects induce ferroelectric polarization in SrTiO$_3$, with the Ti$_\mathrm{Sr}^{\bullet \bullet}$ defect causing a more pronounced spontaneous polarization and higher activation barriers of polarization reversal tha…
In this work we have studied the defect structure and corresponding defect concentration investigations through the theoretical, experimental and computational works on B2-type Fe-Al alloys. We have used ab initio framework in order to investigate the defect structure. To have a proper explanation for high defect concentration in B2-FeAl, we did not confine with point defect, but extend the work on defect complexes. The possible defect formation energies were calculated with the dependence of chemical potential and carefully investigated against supercell size and the effect of magnetism. The …
Dual Role of Nb in Defect-Mediated Strength and Ductility of γ-TiAl Alloys
Institute of High-Performance Computing (IHPC), Agency for Science, Technology and Research (A∗STAR)（高性能计算研究所，科学技术研究局）；State Key Laboratory of Mechanics and Control for Aerospace Structures, Key Laboratory for Intelligent Nano Materials and Devices of Ministry of Education, and Institute for Frontier Science, Nanjing University of Aeronautics and Astronautics（航空航天结构力学与控制国家重点实验室，教育部智能纳米器件重点实验室，前沿科学研究院，南京航空航天大学）；Applied Mechanics and Structure Safety Key Laboratory of Sichuan Province, School of Mechanics and Aerospace, Southwest Jiaotong University（四川省应用力学与结构安全重点实验室，力学与航空航天学院，西南交通大学）；State Key Laboratory of Structural Analysis, Optimization and CAE Software for Industrial Equipment, Department of Engineering Mechanics, Dalian University of Technology（工业装备结构分析优化及CAE软件国家重点实验室，工程力学系，大连理工大学）；Department of Mechanical Science and Bioengineering, Graduate School of Engineering Science, The University of Osaka（机械科学与生物工程学科，工程学研究院，大阪大学）
Formation energies, charge transitions levels, and quasiparticle defect states of the tellurium antisite $(\text{Te}_\text{Cd})$ in CdTe are addressed within the DFT${0.05cm}+{0.05cm}$\emph{GW} formalism. We find that $(\text{Te}_\text{Cd})$ induces a (+2/0) deep level at 0.99 eV above the valence band maximum, exhibiting a negative-U effect. Moreover, the calculated zero-phonon line for the excited state of $(\text{Te}_\text{Cd})^0$ corresponds closely with the $\sim$1.1 eV band, visible in luminescence and absorption experiments. Our results differ from previous theoretical studies, mainly d…
The presence of a point defect typically breaks the stoichiometry in a semiconductor. For example, a vacancy on an A-site in an AB compound makes the crystal B-rich. As the stoichiometry changes, so do the chemical potentials. While the prevalent first-principles methods have provided significant insight into characters of point defects in a transparent manner, the crucial connection between crystal stoichiometry and chemical potentials is usually not made. However, ad hoc choices for chemical potentials can lead to nonphysical negative formation energies in some Fermi level ranges, along with…
Defect engineering using self-doping or creating vacancies in polycrystalline oxide based materials has profound influence on optical absorption, UV photo detection, and electrical switching. However, defects induced semiconducting oxide devices show enhancement in photo detection and photosensitivity, and hence still remain interesting in the field of optoelectronics. In this study, defect engineering in semiconducting oxide materials is discussed along with their roles in optoelectronic device-based applications. Theoretical investigations have been done for identifying defect states by perf…
Ordering of mobile defects in functional materials can give rise to fundamentally new phases possessing ferroic and multiferroic functionalities. Here we develop the Landau theory for strain induced ordering of defects (e.g. oxygen vacancies) in thin oxide films, considering both the ordering and wavelength of possible instabilities. Using derived analytical expressions for the energies of various defect-ordered states, we calculated and analyzed phase diagrams dependence on the film-substrate mismatch strain, concentration of defects, and Vegard coefficients. Obtained results open possibiliti…
A thin oxide layer protects metals from electrochemical corrosion and hence the stability of this oxide layer is crucial for corrosion resistance of metals. The dynamics of cationic and anionic point defects which are injected into this oxide layer at the metal-oxide-layer interface and the oxide-layer-environment interface during electrochemical processes determine the stability of this oxide-layer. The point defect model originally advanced by Digby Macdonald and perfected into a correct and complete theory by Bosco Emmanuel provides a concrete basis for investigating this stability question…
The dielectric breakdown at metal-oxide interfaces is a critical electronic device failure mechanism. Electronic tunneling through dielectric layers is a well-accepted explanation for this phenomenon. Theoretical band alignment studies, providing information about tunneling, have already been conducted in the literature for metal-oxide interfaces. However, most of the time materials were assumed defect free. Oxygen vacancies being very common in oxides, their effect on band lineup is of prime importance in understanding electron tunneling in realistic materials and devices. This work explores …
A deep understanding of defects is essential for the optimisation of materials for solar energy conversion. This is particularly true for metal oxide photo(electro)catalysts, which typically feature high concentrations of charged point defects that are electronically active. In photovoltaic materials, except for selected dopants, defects are considered detrimental and should be eliminated to minimise charge recombination. However, photocatalysis is a more complex process where defects can play an active role, for example, by stabilising charge separation and mediating rate-limiting catalytic s…
Using density functional theory, we calculated the formation energy of native point defects (vacancies, interstitials and antisites) in MAX phase $Ti_2GeC$ and $Ti_3GeC_2$ compounds. Ge vacancy with formation energy of 2.87 eV was the most stable defect in $Ti_2GeC$ while C vacancy with formation energy of 2.47 eV was the most stable defect in $Ti_3GeC_2$. Ge vacancies, in particular, were found to be strong phonon scattering centres that reduce the lattice contribution to thermal conductivity in $Ti_2GeC$. In both compounds, the reported high thermal and electrical conductivity is attributed …
Stability of oxygen point defects in Ruddlesden-Popper oxides (La$_{1-x}$Sr$_{x}$)$_{2}$MO$_{4{\pm}δ}$ (M = Co, Ni, Cu) is studied with density functional theory calculations to determine their stable sites, charge states, and energetics as functions of Sr content ($x$), transition metal (M) and defect concentration (δ). We demonstrate that the dominant O point defects can change between oxide interstitials, peroxide interstitials, and vacancies. Generally, increasing $x$ and atomic number of M stabilizes peroxide over oxide interstitials, as well as vacancies over both peroxide and oxide inte…
